The non-invasive methods for measuring blood pressure in prior art employ the oscillometric method on the basis of cuff PW. The non-invasive method for measuring the blood pressure of human body mainly comprises an auscultatory method using Korotkoff sound (simply referred as Korotkoff sound method) and proportional coefficient method using cuff oscillating wave (simply referred as oscillometric method).
Korotkoff Sound method means that, the experienced medical personnel use a stethoscope, a mercury pressure gauge, a cuff and an inflation/deflation bag, wind the cuff around a proper position of an upper arm of the living body on examination, press the stethoscope close to the humeral artery, inflate the cuff via the inflation/deflation bag to increase the pressure until the block of the blood flow, and then release the cuff pressure step by step via the inflation/deflation bag to recover the blood flow in the arm. During the deflation process, the experienced medical personnel can hear the Korotkoff sound through the stethoscope and mercury pressure gauge, which changes from small to large and then from large to small due to the blood flow pulse in the artery of the arm during the deflation process, and thereby can determine the systolic pressure and diastolic pressure. While the software in the system performs the inflation and deflation control mentioned above, it also identifies the cuff pressure and PW within the cuff detected in respective steps during the deflation process with the characteristic wave, and recovers the trend envelope curve of the cuff oscillating wave on the basis of such characteristic PW. The recover methods of different facilities may differ from each other. Since the recover method has a direct effect on the formation of the envelope curve, it will to a large extent determine the accuracy of measuring the blood pressure of human body. The method widely used nowadays refers to recovery of the PW amplitude value between steps by linear interpolation method, elimination of abnormal fluctuations by multi-point moving average method, and thereby acquisition of envelope curve of the PW amplitude by linear fitting.
The method for measuring the blood pressure based on oscillometric method is currently applied to most of the electronic apparatus for measuring blood pressure. The basic process of this oscillometric method is quite similar to that of auscultatory method, that is, increasing the cuff pressure by inflation to block the blood flow in the arm, reducing the cuff pressure gradually by deflation to recover the blood flow in the ram, and detecting the static state pressure in the cuff and pressure pulse wave (PPW) due to the pulse of the artery blood. However, the calculation of blood pressure in this measuring apparatus is achieved by the following steps of: detecting the PPW in the cuff generated by the change in artery blood pulse of the arm during the deflation process and the corresponding cuff pressure, wherein a set of PPW with the amplitude varying from small to large and vice versa and a corresponding cuff pressure varying from large to small can be detected; then, taking the cuff pressure corresponding to the maximum value of the PPW as an average pressure and utilizing the empirical value for the amplitude proportional coefficient of the PPW to estimate the systolic pressure directed to a higher cuff pressure and the diastolic pressure directed to a lower cuff pressure (in short, the proportional coefficient method based on cuff oscillating pulse amplitude). The systolic pressure and the diastolic pressure can be obtained by multiplying the maximum value of the PPW by two coefficients smaller than 1.
The electronic apparatus for measuring blood pressure generally comprises the followings: (i) a pressure sensor for detecting cuff pressure and a processing circuit, (ii) a PPW processing circuit based on the change in the cuff pressure, (iii) an overpressure sensor as well as an amplification circuit and a protection processing circuit, (iv) a cuff, a control deflation valve, an air pump, gas path connected to the pressure sensor, and an inflation/deflation control section, (v) A/D converter and a single chip system, and (vi) power supply portion. During the measurement process, the detection of PPW and the cuff pressure can be performed during the deflation stage following inflation or during the inflation stage. During the deflation stage, the deflation may be continuously uniform mode (i.e. uniformly decreasing the pressure by 3-5 mmHg while detecting the PPW at the meantime). Alternatively, the deflation may also be stepwise (i.e. gradually reducing the cuff pressure by a step of 5-10 mmHg, and detecting the PPW at each pressure step), in which the magnitude of stepwise reduction of pressure is determined by the detected amplitude of the PPW. Continuously uniform deflation leads to time increase for blood pressure measurement, and it is hard to overcome an influence on the arm movement and the body position change, which would limit its application. However, stepwise deflation leads to time reduction for blood pressure measurement, and eliminates the disturbance resulting from the arm movement and the body position change, etc., with better anti-disturbance capability. Therefore, stepwise deflation is widely adopted in many blood pressure measurement apparatus.
As to the oscillometric method for non-invasive blood pressure measurement, the blood pressure measurement should be implemented by combination of hardware and software, wherein the hardware portions are mainly responsible for i) the amplification of the cuff pressure and the PPW signal, ii) another independent overpressure protection circuit for the cuff pressure, and iii) digital circuit part. The main drawbacks of the hardware portions are set forth as follows:
A). the detection of the cuff pressure requires a timed operation of zeroing, and at present zeroing is automatically performed under the condition that the cuff is not inflated. However, there exists circuit drift and it can not be ensured that the static pressure in the cuff during multiple blood pressure measurement process always drops to nearly “zero”, which will lead to the following results: i) the possible failure in zero correction, and ii) the pressure value deviation due to the offset of zero point even in case of really zeroing. In short, an adverse effect would be exerted on the accuracy in detecting the cuff pressure.
B). In view of safety, two sets of independent timing system are required during the blood pressure measurement process to ensure the limit of measurement time. The blood pressure measurement module in the prior art generally contains a set of timing system itself, and provides an interface to connect with a timing trigger port provided externally. By means of the timing function of the upper computer, not only another set of independent timing system can be accomplished, but the safety requirement of independent timing can be met as well. However, this blood pressure module does not perform a complete safety independent timing and requires the cooperation of the upper computer, which may cause potential danger for the application of such blood pressure measurement module.
C). Since the amplitude for the PPW generated in the cuff during the actual deflation process increases from small to large, and then decreases from large to small after reaching the maximum of amplitude, an asymmetric and non-linear curve of trend envelope is thus formed. Therefore, there are the disadvantages of fitting with linear trend in that the envelope curve of the PW amplitude can not be correctly recovered, and the average pressure, the systolic pressure and the diastolic pressure can be not calculated accurately.